EF Eridani

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Variable star designation	EF Eridani (EF Eri)
Variable type	AM (AM Herculis star, or polar)
Right ascension (J 2000)	03h 09m 47s
Declination (J2000)	-22° 35.7′
Range (magnitudes)	14.5 - <17.5
Period	irregular

EF Eridani (abbreviated EF Eri, sometimes incorrectly referred to as EF Eridanus) is a variable star of the type known as polars, AM Herculis stars, or magnetic cataclysmic variable stars. Historically it has varied between apparent magnitudes 14.5 and 17.5, although since 1995 it has generally remained at the lower limit. The star system consists of a white dwarf star with a substellar massed former star in orbit.

The substellar mass in orbit around the white dwarf is a star which lost all of its gas to the white dwarf, except what remains, a 0.05 solar mass ball, which is too small to continue fusion, and does not have the composition of a super-planet, brown dwarf, or white dwarf. There is no category for such a stellar remnant.

It is theorized that 500 million years ago, the white dwarf started to cannibalize its partner, when they were separated by 7 million km. As it lost mass, the regular star spiraled inward, until now they are separated by a mere 700,000 km.